LaTeX Error: Unknown graphic extension: .gif

Dear LaTeXers,

I just started a new job as a tech writer, and i'd like to use LaTeX
to write our manuals. So i started to make an example to show my
colleagues how great it is, but i can't include a gif file !! I
promise i looked all morning for an answer on the web...

I (try to) use MiKTex with TeXnikCenter, and compile with "LaTeX =>
pdf" compiler. Here's the bit of code:

\documentclass{book} % Your input file must contain these two
%\usepackage[pdftex]{graphicx} % this i tried but didn't work




and i get the error "LaTeX Error: Unknown graphic extension: .gif".
Mind you, including a pdf file works fine.

Thank in advance,